This document relates to the processing of metals and semi-metals, or metalloids, and their compounds and alloys, but to avoid repetition reference is made only to metals in most instances. The skilled person would readily appreciate, however, that in such cases the term metal should be interpreted to encompass both metals and semi-metals or metalloids.
Many metals form oxides, and some have a significant solubility for oxygen. In many cases, dissolved oxygen is detrimental and therefore needs to be reduced or removed before the metal can be fully exploited for its mechanical or electrical properties. For example, titanium, zirconium and hafnium are highly reactive elements and rapidly form an oxide layer when exposed to oxygen-containing environments, even at room temperature. This passivation is the basis of their outstanding corrosion resistance under oxidising conditions. However, this high reactivity has attendant disadvantages which have dominated the extraction and processing of these metals.
As well as oxidising at high temperatures in the conventional way to form an oxide scale, titanium and other elements have a significant solubility for oxygen and other metalloids (e.g. carbon and nitrogen) which results in a serious loss of ductility. This high reactivity of titanium and other Group IVA elements extends to reaction with refractory materials such as oxides, carbides etc. at elevated temperatures, again contaminating and embrittling the basis metal. This behaviour is extremely deleterious in the commercial extraction, melting and processing of the metals concerned.
Typically, extraction of a metal from its oxide is achieved by heating the oxide in the presence of a reducing agent (the reductant). The choice of reductant is determined by the comparative thermodynamics of the oxide and the reductant, specifically the free energy balance in the reducing reaction. This balance must be negative to provide the driving force for the reduction to proceed.
The reaction kinetics are influenced principally by the temperature of reduction and by the chemical activities of the components involved. The latter is often an important feature in determining the efficiency of the process and the completeness of the reaction. For example, it is often found that although a particular reduction should in theory proceed to completion, the kinetics are considerably slowed down by the progressive lowering of the activities of the components as the reduction progresses. In the case of an oxide source material, this may result in a residual content of oxygen (or other impurity elements which may be present) which can be deleterious to the properties of the reduced metal, for example, by lowering ductility, etc. This frequently leads to the need for further operations to refine the metal and remove the final residual impurities to achieve high quality metal.
Because the reactivity of Group IVA elements is high, and the deleterious effect of residual impurities serious, extraction of these elements is not normally carried out from the oxide but, following preliminary chlorination, by reducing the chloride. Magnesium or sodium are often used as the reductant. In this way, the deleterious effects of residual oxygen may be avoided. This more complex process inevitably leads, however, to higher costs which make the final metal more expensive, limiting its applications and its value to a potential user.
Despite the use of this process, contamination with oxygen still occurs. During processing of metals at high temperatures, for example, a hard layer of oxygen-enriched material is often formed beneath a conventional oxide scale. In titanium alloys this is often called the “alpha case”, from the stabilising effect of dissolved oxygen on the alpha phase in alpha-beta alloys. If this layer is not removed, subsequent processing at room temperature can lead to the initiation of cracks in the hard and relatively brittle alpha-case surface layer. These can then propagate into the body of the metal, beneath the alpha case. If the hard alpha case or cracked surface is not removed before further processing of the metal, or before the fabricated product enters service, there can be a serious reduction in performance, especially of fatigue properties. Heat treatment in a reducing atmosphere is not available as a means of overcoming this problem for Group IVA metals because of the embrittlement of these metals by hydrogen and because the oxide or “dissolved oxygen” cannot be sufficiently reduced or minimised. The commercial costs arising from this problem are significant.
In practice, for example, metal is often cleaned up after hot working by removing the oxide scale by mechanical grinding, grit-blasting, or using a molten salt, and then by acid pickling, often in HNO3/HF mixtures, to remove the oxygen-enriched layer of metal beneath the scale. These operations are costly in terms of loss of metal yield, consumables and not least in effluent treatment. To minimise scaling and the costs associated with the removal of the scale, hot working is generally carried out at as low a temperature as is practical. This, however, reduces plant productivity, and increases the load on the plant due to the reduced workability of the metal at lower temperatures. All of these factors increase the processing costs.
In addition, acid pickling is not always easy to control, either in terms of hydrogen contamination of the metal, which leads to serious embrittlement problems, or in terms of surface finish and dimensional control. The latter is especially important in the production of thin materials such as thin sheet, fine wire, etc.
It is evident therefore, that a process which can remove the oxide layer from a metal and additionally the dissolved oxygen of the sub-surface alpha case, without the grinding and pickling described above, could have considerable technical and economic benefits on metal processing and metal extraction.
Such a process may also have advantages in ancillary steps of the purification treatment or processing of metals. For instance, scrap turnings produced during either the mechanical removal of the alpha case, or machining of a product to finished size, are difficult to recycle due to their high oxygen content and consequent hardness, and the resulting effect on the chemical composition and hardness of the metal into which they are recycled.
Even greater advantages might accrue if a metal which had been in service at elevated temperatures and had therefore been oxidised or contaminated with oxygen could be rejuvenated by a simple treatment. For example, the life of an aero-engine compressor blade or disc made from titanium alloy is constrained, to a certain extent, by the depth of the alpha case layer which forms during fabrication and during service and the consequent dangers of surface crack initiation and propagation into the body of the disc leading to premature failure. In this instance, acid pickling and surface grinding are not possible since a loss of dimension could not be tolerated. A technique which lowered the dissolved oxygen content without affecting the overall dimensions of a component, especially in complex shapes such as blades or compressor discs, would have obvious and very important economic benefits. In an aero-engine, for example, because of the effect of temperature on thermodynamic efficiency, these benefits would be compounded if they allowed the discs to operate not just for longer times at the same temperature, but also possibly at higher temperatures where greater fuel efficiency of the aero-engine can be achieved.
In addition to titanium, a further metal of commercial interest is Germanium, which is a semi-conducting semi-metal, or metalloid, element found in Group IVB of the Periodic Table. It is used, in a highly purified state, in infra-red optics and electronics. Oxygen, phosphorus, arsenic, antimony and other metalloids are typical of the impurities which must be carefully controlled in Germanium to ensure adequate performance. Silicon is a similar semiconducting element and its electrical properties depend critically on its purity content. Controlled purity of the parent silicon or germanium in fabricating devices is fundamentally important to provide a secure and reproducible basis onto which the required electrical properties can be built up in computer chips, etc.
U.S. Pat. No. 5,211,775 discloses the use of calcium metal to deoxidise titanium. Okabe, Oishi and Ono (Met. Trans B. 23B (1992):583, have used a calcium-aluminium alloy to deoxidise titanium aluminide. Okabe, Nakamura, Oishi and Ono (Met. Trans B. 24B (1993):449) describe the removal of oxygen dissolved in solid titanium by electrochemically producing calcium from a calcium chloride melt, on the surface of the titanium-oxygen solid solution. Okabe, Devra, Oishi, Ono and Sadoway (Journal of Alloys and Compounds 237 (1996) 150) have deoxidised yttrium using a similar approach.
Ward et al, Journal of the Institute of Metals (1961) 90:6-12, describes an electrolytic treatment for the removal of various contaminating elements from molten copper during a refining process. The molten copper is treated in a cell with barium chloride as the electrolyte. The experiments show that sulphur can be removed using this process. However, the removal of oxygen is less certain, and the process requires the metal to be molten, which adds to the overall cost of the refining process. The process is therefore unsuitable for a metal such as titanium which melts at 1660° C., and which has a highly reactive melt.
PCT/GB99/01781 describes an electrolytic method, termed electro-deoxidation, for the removal of oxygen and other non-metal species from a sample of a solid metal or metal compound by making the sample the cathode in a calcium chloride melt. Taking as an example the treatment of a metal or metal compound containing oxygen, when a cathodic potential below the potential for deposition of calcium from the calcium chloride was applied, the oxygen in the sample preferentially ionised.